(1) Field of the Invention
This invention relates to a high strength steel plate which is low in yield ratio and improved in strength-elongation balance as well as in stretch flangeability, and a method for manufacturing same. The high strength steel plate of the present invention is particularly suitable for use as a material for the wheel disc and/or wheel rim of a motor vehicle.
(2) Description of the Prior Art
Recently, various attempts are made in order to improve the mileage of motor vehicles, including the reduction of the vehicle body weight which is considered to be most effective. With regard to the reduction of the vehicle body weight, there have thus far been made many proposals concerning the use of high strength steel plates and aluminum alloys along with the reductions in size. Among these proposals, the reduction of the vehicle wheel weight is one of the most effective means for improving the mileage, and the possibilities of application of a high strength steel plate to the wheel rim or disc have been a subject of intensive studies. The high strength steel plates which have been proposed for this purpose include the composite structure steel plate (dual phase steel plate of ferrite+martensite, which is low in yield ratio and has higher elongation as compared with the strength, showing excellent properties in formability and shape fixability. However, the steel plate of this sort is inferior in stretch flangeability so that, if applied to the vehicular wheel disc or the like, it gives rise to problems such as:
(1) the occurrence of cracking at an expanded hole portion in the disc forming operation; or PA1 (2) the occurrence of cracking at an expanded hole portion in the fatigue test or in the running test. PA1 (i) hot rolling the steel at a fininshing rolling temperature above Ar.sub.3 point, followed by cooling the hot rolled steel from the finishing rolling temperature to a temperature range between point Ar.sub.3 and point Ar.sub.1 at an average cooling speed of 3.degree.-70.degree. C./sec, PA1 (ii) hot rolling the steel at a finishing rolling temperature above Ar.sub.3 point, followed by cooling the hot rolled steel from the finishing rolling temperature to a temperature range between point Ar.sub.3 and point Ar.sub.1 at an average cooling speed of 3.degree.-70.degree. C./sec and then by air cooling ar slow cooling for 2-20 seconds from the temperature range of Ar.sub.3 to Ar.sub.1, and PA1 (iii) hot rolling the steel at a finishing rolling temperature range of Ar.sub.3 to Ar.sub.1, followed by air cooling or slow cooling the hot rolled steel for 2-20 seconds from the temperature range of Ar.sub.3 to Ar.sub.1, thereafter PA1 (2) cooling the steel thus subjected to hot rolling-cooling treatment to a temperature below 550.degree. C. at an average cooling speed not lower than 20.degree. C./sec, and PA1 (3) taking up the cooled steel.
The present inventors have studied in detail the relationship between the steel structure and stretch flangeability for the improvement thereof, and as a result found that a steel plate of a ferrite+bainite structure is superior to a dual phase steel plate of ferrite+martensite in the stretch flangeability, as disclosed in our Japanese Patent Application Nos. 54-171594 and 55-33155. However, the steel plate of the ferrite+bainite structure has a drawback that it is inferior in the strength-elongation balance.
Further, the wheel rim requires the resistance weldability in addition to the stretch flangeability which is required by the wheel disc. Another problem which is encountered in applying a high strength steel plate to the wheel rim is the cracking which takes place at a high rate in the roll-forming operation subsequent to the flash butt welding, the cracks occurring at a rate as high as about 50% in the thermally affected zones in the forming stage. Such a high rate of cracking is detrimental to actual applications.